THE RECENT PROGRESS OF PHYSICAL SCIENCE. 745 



wlien tlie changes are sufficiently rapid, the phenomena are governed 

 much more by induction, or electric inertia, than by mere resistance. 

 On this principle much may be explained that would otherwise seem 

 paradoxical. To take a comparatively simple case, conceive an elec- 

 tro-magnet wound with two contiguous wires, upon which acts a given 

 rapidly periodic electro-motive force. If one wire only be used, a cer- 

 tain amount of heat is developed in the circuit. Suppose now that the 

 second wire is brought into operation in parallel — a proceeding equiv- 

 alent to doubling the section of the original wire. An electrician, 

 accustomed only to constant currents, would be sure to think that the 

 heating effect would be doubled by the change, as much heat being 

 developed in each wire separately as was at first in the single wire. 

 But such a conclusion would be entirely erroneous. The total current, 

 being governed practically by the self-induction of the circuit, would 

 not be augmented by the accession of the second wire, and the total 

 heating effect, so far from being doubled, would, in virtue of the 

 supei'ior conductivity, be halved. 



During the last few years much interest has been felt in the reduc- 

 tion to an absolute standard of measurements of electro-motive force, 

 current, resistance, etc., and to this end many laborious investigations 

 have been undertaken. The subject is one that has engaged a good 

 deal of my own attention, and I should naturally have felt inclined to 

 dilate upon it, but that I feel it to be too abstruse and special to be 

 dealt with in detail upon an occasion like the present. As regards re- 

 sistance, I will merely remind you that the recent determinations have 

 shown a so greatly improved agreement that the Conference of Elec- 

 tricians assembled at Paris, in May, have felt themselves justified in 

 defining the ohm, for practical use, as the resistance of a column of 

 mercury of 0° C, one square millimetre in section, and 106 centi- 

 metres in length — a definition differing by a little more than one per 

 cent from that arrived at twenty years ago by a committee of this 

 Association. 



A standard of resistance once determined upon can be embodied 

 in a "resistance-coil," and copied without much trouble, and with 

 great accuracy. But, in order to complete the electrical system, a 

 second standard of some kind is necessary, and this is not so easily 

 embodied in a permanent form. It might conveniently consist of a 

 standard galvanic cell, capable of being prepared in a definite manner, 

 whose electro-motive force is once for all determined. Unfortunately, 

 most of the batteries in ordinary use are, for one reason or another, 

 unsuitable for this purpose, but the cell introduced by Mr. Latimer 

 Clark, in which the metals are zinc in contact with saturated zinc sul- 

 phate and pure mercury in contact with mercurous sulphate, appears 

 to give satisfactory results. According to my measurements, the elec- 

 tro-motive force of this cell is 1'43.5 theoretical volt. 



We may also conveniently express the second absolute electrical 



